Thyroglobulin is the major protein of the thyroid. It provides a matrix for the synthesis of the thyroid hormones and a vehicle for their subsequent storage. Our long range objectives are to define its role in the synthesis and release of thyroid hormones, and to investigate the possibility that variuations in its structure may lead to goiter in humans. In work to date we have found that reduced throglobulin from a number of species contains discrete thyroxine-rich peptides of small size, and the distribution of iodine among them is influenced by the amount of iodine available and by TSH. We have also described a number of lysosomal proteases which digest thyroglobulin and thereby effect the release of hormone. In addition, we have found variations in the structure of thyroglobulin from normal thyroids, and even more variation in that from goiters and thyroid cancer. We plan further experiments as follows: (1) To define the origins of the thyroxine-rich peptides of reduced thyroglobulin, we will study their amino acid incorporation in vitro, the effects of time, iodine deficiency, and TSH in vivo, the variation among normal and goitrous human glands and among different vertebrate species, and the site from which the outer ring of thyroxine is donated. (2) We plan to sequence key parts of the thyrozine-rich peptides to identify the chemical features important to hormone synthesis. (3) The physiological steps in Tg breakdown and hormone release will be evaluated by further purification and characterization of thiol endopeptidases and of exopeptidases, by characterization of the thyroxine-rich fragments produced by these proteases, by investigation of TSH's role in the rate of proteolysis, and by in vitro models for the physiological hydrolysis of thyglobulin.